This invention relates to packaging materials useful for microwave cooking applications, and particularly to packaging material which will brown and crispen food items and which provide a selectively releasable seal around such food items.
There has been much interest recently in packaging materials which aid in browning and crispening of food items in a microwave oven. U.S. Pat. No. 4,267,420, to Brastad, discloses a food item wrapped with plastic film having a very thin coating thereon. The film conforms to a substantial portion of the food item. The coating converts some of the microwave energy into heat which is transmitted directly to the surface portion of the food so that a browning and/or crispening is achieved.
U.S. Pat. No. 4,676,857, to Scharr, discloses a microwave heating material and method for its preparation. A preselected metallized pattern, such as dots, spirals, or circles, is disposed on at least a portion of a dielectric material. The dielectric material may be in the form of a flexible wrap.
Other inventions have used the fact that various polymeric materials lose strength at elevated temperatures to perform useful packaging functions. U.S. Pat. No. 4,404,241, to Mueller et al., discloses a microwave package with a means for venting vapor. The vent is in the form of an aperture in the multilayer sheet which forms the package, and is covered with a continuous sealing layer of an extrudable hot melt material. When this material is subjected to slight pressure in combination with heat, softening and flow occurs at temperatures effective to permit venting of steam or other vapor without sufficient pressure build-up to distort the package.
U.S. Pat. No. 4,561,337, to Cage et al, discloses a bag and a mixture of edible popcorn ingredients suitable for use in microwave ovens. Portions of the panels of the bag contain a coating that is sensitive to pressure and heat, forming a seal along the top edge of the panels. The seal has sufficient strength to withstand the internal steam pressure generated by the moisture content of the kernels for at least one-half of the popping process. Preferably, the bag will vent at the top seam before the process is completed to allow steam to escape.
In spite of significant efforts in formulating packaging materials suitable for microwave cooking applications, there remain foods which are difficult or impossible to prepare in a microwave oven. Examples include "puff pastries" such as filled turnovers, which must both rise and be browned during cooking. When such foods are cooked in a microwave oven, they may fail to rise or may rise only irregularly, and may not brown. The packages in the prior art are not designed to allow such foods to rise. They generally require close contact between the food and the microwave susceptive packaging material, and thus constrain the food. If a gap were to be left between the food and the film, large enough to permit the food to expand, there would then not be sufficient heat transfer between the film and the food for proper cooking or browning.
The present invention, in contrast, provides packages for use in microwave ovens which allow the proper browning and shaping of foods which rise upon cooking. The present invention further provides packages which allow proper browning of foods as well as venting of the steam or vapor generated from cooking.